1. Technical Field
This invention relates to a non heat-treated steel which exhibits high levels of strength, toughness and machinability without being heat-treated prior to cutting after hot rolling, and which is suitable as a machine structural steel used by cutting after hot rolling, and after hot or cold working if required.
2. Background Art
A machine structural alloy steel SCM435 or SCM440 as specified by JIS G4105 has hitherto been used for making machine structural, or automobile parts of which high levels of strength and toughness are required. These parts are usually manufactured by (1) shaping by rolling, and further by hot or cold working, if required, (2) heat treatment, such as hardening and tempering, for imparting strength and toughness to steel, and then, (3) cutting.
The structural alloy steel as mentioned above calls for heat treatment as a step (2) for achieving the strength and toughness which are required.
The heat treatment, or heat treatment requires a long time and a high cost. If heat treatment can be omitted, it is possible to realize a great reduction of cost and also a reduction of energy consumption, so various proposals have been made for that purpose.
There has, for example, been proposed a non heat-treated steel of the ferrite-pearlite type obtained by adding about 0.10 wt % of vanadium to medium-carbon steel containing maganese having a carbon content of 0.3 to 0.5 wt %. Vanadium carbonitride is precipitated during cooling after hot rolling to strengthen the ferrite, while the strength of the pearlite is relied upon for raising the strength of the steel as a whole.
Japanese Patent Publication No. Hei 6-63025 and Japanese Patent Application Laid-Open No. Hei 4-371547 disclose as hot forging steels non heat-treated steels of the bainitic or martensitic type which are produced by adding manganese, chromium, vanadium, etc. to low carbon steels having a carbon content of, say, 0.05 to 0.3 wt %.
Referring to the former non heat-treated steel of the ferrite-pearlite type, however, it is difficult to produce steel having both high tensile strength and high toughness. The carbon present in the amount of 0.3 to 0.5 wt % as the cementite in the ferrite raises the strength of steel, but lowers its toughness. Moreover, as the vanadium carbonitride precipitated in the ferrite-pearlite structure is relied upon for raising the strength of steel, only a limited range of cooling rate is applicable to produce steel which is stable in properties. It is necessary to control its cooling rate after rolling, or hot working, and its manufacturing process is accordingly complicated. Cold working, such as cold forging, makes it possible to raise the strength of steel without increasing its carbon content, but makes it impossible to obtain steel which is comparable in toughness to heat-treated steel.
The latter non heat-treated steel of the bainitic type proposed in Japanese Patent Publication No. Hei 6-63025 is inferior in yield strength to heat-treated steel when it is only hot forged. In order to raise its yield strength, it is essential to subject it to aging treatment at a temperature of 200.degree. C. to 600.degree. C. after hot forging and allow it to cool (in the air). Therefore, it is impossible to achieve a reduction of energy consumption as one of the merits of non heat-treated steel. No reduction of energy consumption can be achieved by the process for manufacturing high-strength and high-toughness non heat-treated steel proposed in Japanese Patent Application Laid-Open No. Hei 4-371547, either, since it calls for tempering. If this kind of steel is used for making a small part, it is easy to obtain a satisfactorily high level of toughness, since it is possible to employ a high cooling rate after hot forging. When it is used for making a large part, however, it is impossible to obtain a satisfactorily high level of toughness steadily unless the cooling rate after hot forging can be so controlled as to be sufficiently high.
Japanese Patent Applications Laid-Open Nos. Hei 8-144019 and Hei 9-111336 disclose low carbon steels containing copper and boron which exhibit a satisfactorily high level of toughness even if a low cooling rate may be employed. It is, however, often the case that the manufacture of a machine structural part includes cutting after various steps of working, such as rolling and forging, and heat treatment, as stated before. It is, therefore, essential for any industrially useful material to have not only high strength and high toughness, but also high machinability. The low carbon steels containing copper and boron are, however, not intended for achieving any practically acceptable level of machinability as required for making a machine structural part, since they are mainly intended for achieving high toughness without being heat-treated.
Moreover, Japanese Patent Application Laid-Open No. Sho 60-92450 discloses steel having its strength improved by the precipitation of copper. This steel is made by adding 0.5 to 2 wt % of copper to nitriding steel, and has its strength increased by the precipitation of copper during its nitriding. Its disclosure does not discuss anything about machinability, either. Moreover, as the steel has a carbon content of 0.05 to 0.3 wt %, its tensile strength is greatly lowered by mass effect when applied to a material, or part having a large diameter or size and thus a low cooling rate.
It is, therefore, an object of this invention to provide a non heat-treated steel which can be used as hot or cold worked, and yet exhibits high strength, high toughness and excellent machinability even when used for making a large structural part.